ACES is a multicenter, collaborative clinical trial of the antibiotic, Azithromycin, versus placebo for patients with documented coronary artery disease. The purpose is to determine if this antibiotic treatment can limit subsequent heart attacks and mortality, presumably by eradicating the bacterium, Chlamydia pneumoniae, which has been found in the atherosclerotic plaque. Subjects take Azithromycin 600 mg or placebo once weekly for 1 year. Follow-up is by telephone for 5 years. One fourth of patients return at 6, 12 and 18 months for blood testing for Chlamydia antibodies.